When gases are combined they nearly always exhibit substantially homogeneous distribution within a confined space. Even unconfined spaces such as the atmosphere contain a substantially homogeneous mixture of gases. It is often desirable to have gases collected in pure or substantially pure form. One way to collect such gases is to separate them from a homogeneous mixture of gases where one desired gas is mixed with other gases. For instance, it is often desirable to have oxygen concentrated into a substantially pure form by separation of the oxygen from air.
Air in the atmosphere is a substantially homogeneous mixture of approximately 79% nitrogen, 20% oxygen and 1% argon. Air also includes water vapor to varying degrees depending on the humidity of the air. Air also includes a fraction of a percent of carbon dioxide and trace amounts of other gases such as hydrogen, helium, other noble gases and small trace amounts of gaseous compounds.
One known technique for separating oxygen from air (or separating other desirable gases from a gas mixture) is to take advantage of the different condensation points for different gases at which the gases condense into a liquid. Such “liquefaction” is particularly effective when the gases in the gas mixture have widely different condensation points, and particularly when at least one of the condensation points is near ambient temperature. For instance, a condenser for condensing water vapor out of air is effectively operated, often with little or no power input, to remove large portions of gaseous water (i.e. steam or water vapor) from air.
However, when gases to be separated have similar condensation temperatures or the condensation temperatures are significantly lower than ambient conditions, significant power and potentially elaborate machinery is required for effective gas separation. When oxygen is to be separated from air, such difficulties are encountered. Oxygen and nitrogen have quite similar condensation temperatures and these condensation temperatures are significantly lower than ambient conditions (i.e. −320° F. for nitrogen and −297° F. for oxygen) Thus, liquefaction for effective separation of oxygen from air requires significant power input and elaborate machinery, making such liquefaction undesirable in many instances.
Another widely used technique for air separation (especially when the desired product is nitrogen) is membrane technology. Membrane technology can also be used to concentrate oxygen, but is generally only used when the desired product is nitrogen because it is difficult to get the purity range generally required for oxygen, using membrane technology. In most cases where membrane technology is used, the argon component of the air will stay with the nitrogen while the carbon dioxide and the water will stay with the oxygen.
Another technique for separating gases from a gas mixture is to utilize the unique properties or certain materials which preferentially adsorb one gas over another. For instance, it is known to utilize molecular sieve as an adsorbent which preferentially adsorbs nitrogen over oxygen. When air is passed through a bed of such an adsorbent material, the nitrogen is adsorbed onto the surface of the adsorbent material. Remaining portions of the air are substantially entirely oxygen. Allowing pressure in the bed to swing provides for periods of desorption to repeat the process. Such adsorbent material also adsorbs carbon dioxide and water vapor. While argon is not typically adsorbed and so remains with the oxygen, oxygen can often be effectively utilized even when the argon from the original air gas mixture is still present.
Such pressure swing adsorption systems can be divided into two general types including pressure swing adsorption (PSA) and vacuum swing adsorption (VSA). The primary difference between PSA and VSA is the pressure at which the adsorber material is caused to desorb the gaseous molecules or compounds which had previously adsorbed, to refresh the adsorber material. With PSA, adsorption occurs at a pressure above atmospheric pressure and desorption occurs at a lower pressure, typically at or near atmospheric pressure. With VSA, adsorption occurs at or above atmospheric pressure and desorption occurs below atmospheric pressure in at least a partial vacuum.
Prior art VSA systems known in commercial use are typified by systems such as that provided by Praxair, Air Products Company, Pacific Consolidated Industries and others. An exemplary system is described in U.S. Pat. No. 4,194,890 to McCombs. In such systems, typically one or more adsorption beds are provided. Often two beds are used. In a two bed system one bed adsorbs the nitrogen and other undesirable gasses the other will be in the desorb (refresh) process
The simplest example of current art is the case of a single bed VSA system. In these single bed systems the blower rotates in one direction. Air is fed to the bed from the pressure side of the blower. When the bed is saturated with nitrogen, valves are then actuated to change the inlet of the bed from the outlet side of the blower to the inlet side of the blower. Thus the same blower charges the bed with process air and creates the vacuum to desorb the bed.
In pressure swing systems, (PSA and VSA), a buffer tank is usually included so that a constant supply of oxygen can be provided. In most cases, a heat exchanger (or other method of cooling the gasses) is used between the blower and the adsorption bed to remove some of the heat generated in the blower. Almost always, the PSA or the VSA process is used to deliver oxygen. This invention describes the adaptation of the pressure swing process to produce nitrogen.